The big box time interval on a standard EKG represents how many seconds?

• A. 0.04 seconds
• B. 0.5 seconds
• C. 0.2 seconds
• D. 1.0 seconds

Answer: (C)

Understanding ECG timing relies on the paper speed. On a standard ECG, the paper moves at 25 mm per second. Each small box is 1 mm, which represents 0.04 seconds. A large box is five small boxes, so it spans 5 × 0.04 = 0.20 seconds. Therefore, the interval represented by one big box is 0.2 seconds. This is why 0.2 seconds is the correct choice. The other options correspond to either a single small box (0.04 s), or a longer segment that doesn’t match a single big box (1.0 s is five big boxes, 0.5 s isn’t a standard box interval).